The present invention relates generally to an actuator system of the type in which an output shaft is spring returned to a rest position and, on command, is driven to and held at a different position by an electric motor. More particularly, the invention relates to such an actuator system employing a drive circuit and method which reduces power supplied to the motor when the output shaft is stalled away from its rest position.
It is well known, particularly in heating, ventilating and air conditioning (HVAC) damper applications, to employ actuators of the type having an output shaft which is driven in one direction to a desired position and held in that position by an electric motor, and returned in the opposite direction to a rest position by a spring when the motor is not energized. The motor may also serve to govern the speed with which the spring returns the actuator output shaft to its rest position.
Depending on the type of motor used, the motor may offer more or less minimum resistance to operation of the spring return mechanism. This resistance is manifested as a torque in addition to the torque required for returning the damper or other load to its rest position which must be provided by the spring. The speed with which the motor can operate the load is determined by the power output of the motor which is transmitted to the load by a torque multiplying gear train. The resistance or load provided by the motor on the spring in returning the actuator output shaft to its rest position typically increases with increased power output capability of the motor. Thus, it is apparent that optimizing the actuator system for speed of operation and size of controlled load requires careful balancing of the motor output power capability, gear train input/output ratio and return spring strength.
One function of dampers in certain HVAC systems is to provide smoke and fire control. It has become a requirement that actuators used in smoke and fire control applications be capable of operation at an elevated temperature of, for example, 350.degree. F. Operation at elevated temperature introduces additional complications and places additional demands on the actuator system. More specifically, magnetic circuit performance is generally adversely affected by elevated temperature, thus decreasing electric motor power output for a given energization voltage. A requirement for operation at elevated temperatures also places limitations on the electrical circuit design, which effectively precludes use of electronically commutated motors. Finally, elevated temperature application requirements restrict the choice of acceptable materials and lubricants, effectively precluding the use of many plastics and wick-type lubrication systems.
Apart from the foregoing considerations, it is desirable to minimize the energy consumed by the actuator system. In addition to reducing energy costs, this reduces the power handling requirements of circuit components which supply energization current to the motor, and reduces the power required to be dissipated by the motor, thereby permitting use of a motor of smaller size and increasing its life.
The applicant has achieved many of the objectives and operating characteristics indicated as desirable in the foregoing discussion by devising an actuator designed around a DC brush commutated motor. The characteristics of such a motor are used to maximum advantage by providing a unique drive circuit and method of energization which alters the average voltage at which current is supplied to energize the motor based on the actuator operating mode and environmental conditions. This approach has permitted the applicant to provide a fast acting two position spring return actuator designed to be directly coupled to a load. The actuator requires less operating power than conventional actuator designs and is capable of operation at elevated temperatures.